1. Field of the Invention
The invention relates to an image forming apparatus having copying function.
2. Related Background Art
In recent years, in order to inhibit illegal copy or falsification of manuscript with high secret, a technology to bury specific addition information serving as electronic watermark into manuscript data i.e. image data is put to practical use.
Such invention utilizing the electronic watermark technology is stated in, for example, patent document 1. The invention stated in the patent document 1 buries limitation information which is used to limit copy times and serves as electronic watermark into image data, and prohibits copy exceeding the copy times predetermined by the limitation information.
Patent document 1: Japan patent publication 2005-229176.
However, because the manuscript is repeatedly copied or it is dirtied or damaged, there is a possibility that the electronic watermark information buried in the manuscript degrades so that the electronic watermark information can not be correctly read. In the case, in the invention stated in the patent document 1, because the electronic watermark information can not be analyzed, there is a danger that the manuscript is copied without appropriately performing copy prohibition. That is, because the electronic watermark information can not be analyzed, illegal copy or falsification of manuscript can not be judged. Thus, it is difficult to keep the manuscript secret.